In semiconductor IC fabrication, devices such as component transistors are formed on a semiconductor wafer or substrate, which is typically made of silicon. Metallic interconnect lines, which are etched from a metal layer disposed above the wafer, may then be employed to couple the devices together to form the desired circuit. These metal layers are typically made of aluminum, copper, or an aluminum alloy. Conventional metal etchants usually contain chlorine (Cl.sub.2), which is highly selective for aluminum, and etchants include, for example, BCl.sub.3 /Cl.sub.2, HCl/Cl.sub.2, BCl.sub.3 /Cl.sub.2 /SF.sub.6.
The etched layer stack is then subject to a strip process to remove the photoresist that was applied prior to the metal etch to cover areas of the layer stack which are not to be etched. Stripping may be conducted with a plasma asher. In this device, O.sub.2 is excited into a plasma which dissociates the O.sub.2 into various oxygen radical and ion species that oxidize (ash) the photoresist.
The introduction of highly reactive fluorine-containing gases into an oxygen plasma not only increases the photoresist ash rate but also enhances polymer removal. Higher ash rates are evident even with the addition of a small amount of CF.sub.4 to an O.sub.2 -based plasma. The reason is that CF.sub.4 significantly increases the atomic oxygen concentration which enhances the ash rate of photoresist. Atomic fluorine is also known as a very effective chemical species for removing the polymers formed during the metal etch process. Since these composite polymers are highly non-volatile and etch resistant, O.sub.2 -based plasmas employing etchant gas such as, for example, O.sub.2, O.sub.2 /N.sub.2, O.sub.2 /H.sub.2 O, O.sub.2 /N.sub.2 /H.sub.2 O are ineffective for removing these polymers. Often they cannot be completely removed even by the aggressive chemical solvents. The composite polymer or sidewall polymer typically forms on the vertical walls of the etched layer stack. These polymers are composite byproducts comprising carbon (e.g., from the photoresist), metal, oxides, aluminum silicate, and fluorocarbons. Composite polymers form fences against the via walls that are difficult to remove.
Despite the advantages, there is a major drawback associated with fluorine chemistry. Due to the aggressiveness nature of highly reactive fluorine species, CF.sub.4 added O.sub.2 plasma not only etches targeted photoresist but it also attacks barrier and underlying materials, e.g., Ti, TiN, TiW, and SiO.sub.2. Excess loss of these materials results in significant reduction in photoresist etch selectivity. Thus, removing the photoresist and composite polymer with minimum undercutting remains a key factor for a successful process development.